There is an increasing need for the reduction of weight and the reduction in cost for products made from aluminum brazing sheet, particularly for brazing sheet used in heat exchangers, particularly in automotive applications. Brazing sheet products that exhibit higher-post braze yield strengths are desirable, as these high-strength products allow automotive engineers to downgauge. In short, a high strength brazing sheet product would allow the heat exchanger to be made from a thinner and, therefore, lighter brazing sheet, with corresponding weight savings in the overall automotive design.
In addition, it is equally important that the brazing sheet or plate product have adequate corrosion resistance as well as adequate brazeability to allow the heat exchanger manufacturer to reliably braze the heat exchanger.
Ideally, variants of the products also must be brazeable by a variety of brazing methods, most notably, vacuum and flux-based (e.g. CAB or Nocolok™) brazing processes, to have as wide an application as possible.
Although products which exhibit a recovered, but not recrystallized, microstructure are highly desirable from a post-braze yield strength perspective, it is well known that these microstructures are highly susceptible to localized erosion during the brazing cycle. Non-homogenized 3xxx cores, in O-temper, are known to be sensitive to core erosion during brazing. Core erosion is localized melting of the core alloy in contact with the molten 4xxx cladding and generally is deleterious to corrosion resistance and cladding flow (i.e., brazeability). Localized erosion typically results from enhanced Si diffusion from the 4xxx cladding alloy into the underlying base metal in contact with the 4xxx cladding alloy. The dislocation networks (e.g., sub-grain boundaries) present in recovered, but unrecrystallized, microstructures result in demonstrably higher diffusivities for Si. The enhanced mobility of Si in the presence of a fine network of interlacing dislocations results in high local Si concentrations, which, in turn, result in localized melting of the metal in contact with the 4xxx cladding alloys during the brazing cycle. This localized melting of the core alloy enriches the cladding with aluminum, and changes in-situ the cladding alloy's composition and its flow properties. Localized melting can also alter the surface topography of the metal, which generally retards 4xxx cladding flow during the brazing cycle and results in poor brazeability. Lastly, this localized ingress of Si into the core can result in an increased susceptibility to localized corrosion.